Understanding the Role of Structural Characteristics in the Catalytic and Mechanical Properties of Metal–Organic Frameworks

Redfern, Louis

doi:https://doi.org/10.21985/n2-mrfc-9116

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Understanding the Role of Structural Characteristics in the Catalytic and Mechanical Properties of Metal–Organic Frameworks

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It is said across numerous disciplines, from biology to architecture to software engineering, that “form ever follows function.” This adage highlights the intimate relationship between structural characteristics and functional properties in many disparate fields of work. Here, we discuss how the catalytic activity and compressibility of metal–organic frameworks (MOFs) are closely linked to the structure of each framework. The establishment of structure-property relationships is a central strategy in chemistry to enable the rational design of compounds and materials, e.g. drug discovery or catalyst development. The research discussed in this thesis describes efforts to understand the relationships between MOF structures and their catalytic and mechanical properties. To probe catalytic activity, we studied composite materials comprised of copper nanoparticles (CuNPs) bound within the pores of MOFs as catalysts for the hydrogenation of acetylene. Throughout this work, we synthesized and thoroughly characterized three such composites using synchrotron X-ray scattering and diffraction techniques, along with spectroscopic methods involving carbon monoxide as an infrared probe molecule. By systematically decreasing the pore diameter of the parent MOF, we find that the size of the embedded CuNPs correspondingly decreases, suggesting that the framework pores can act as a template to control the growth of these particles. This promises to be a powerful strategy for the formation of nanoparticles with fine-tuned control over their size. We demonstrate that changes to the CuNP size has a significant impact on the reactivity and selectivity of these composite materials for the hydrogenation of acetylene to form ethylene—an essential chemical feedstock in modern society—without overreduction to ethane. Interestingly, we find that hydrogenation is disfavored in the materials with smaller CuNPs compared to those with large particles, but that C–C coupling reactions to form four-carbon olefins are more prevalent in the presence of the smaller CuNPs. This wide range of reactivity highlights the importance of control over structural properties in catalyst design. In addition to catalytic functionality, we seek to understand the structural characteristics that influence the mechanical properties of MOFs, such as their compressibility under high pressures. To elucidate these relationships, we designed a series of experiments in which structural properties, e.g. the length of the organic linker, were incrementally varied while holding other parameters constant. We monitored the unit cell volume of each MOF in these series as a function of pressure, providing insight into the compressibility of the materials. By comparing how structural properties, such as density and node-to-node distance, influence compression, we find that void fraction correlates well with the bulk modulus (i.e. the resistance to compression under hydrostatic pressure) for seven MOFs belonging to two distinct topological families. Further investigation reveals that distortions of the organic linker at ambient conditions can significantly diminish the mechanical stability of MOFs. While these results implicate the organic linker as a major contributor to mechanical stability, we find that alterations to the inorganic node also influence the compressibility of the framework. We show that identical MOFs with Ce6 nodes exhibit distinctive behavior under pressure when compared to their Zr-derived analogues, as evidenced by X-ray diffraction, Raman spectroscopy, and density functional theory simulations. Together, these results emphasize the need for rigorous, systematic studies to establish structure-property relationships.

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